Radio broadcasting equipment



FIPSIGZ OR 195739984 Feb. 23 v. .,573,984

v J'. P. MAXFELD I RADIO BROADCASTING EQUIPMENT I -fyfflb Filed June 9, 1922 2 Sheetsheet 1 H37. f. mi '72. )(ff X rMv Feb. 2s ,1926. 1,573,984

. J. P. MAXFIELD RADI 0 BROADCASTI NG EQUIPMENT Filed June 9. 1922 2 Sheets-Sheet 2 Patented Feb. 23, x1926.

UNITE STATES PATE T OFFER.

JOSEPH P. TEAXFIELD, OF MAPLEWOOD, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TRIO COMPANY, INCORPORATED, OE NEW YORK, N. Y., A CORPORATION OF NEW YORK.

RADIO BROADCASTING EQUIPMENT.

Application filed June 9,

To all 'whom t may concern:

Be it known that I, JOSEPH P. MAXFIELD, a citizen of the United States of America, residing at Maplewood, in the county of Essex and State of New Jersey, have invented certain new and useful Improvements in Radio Broadcasting Equipment, of which the following is a full, clear, concise, and exact description.

This invention relates to the broadcasting of intelligence, such as speech, music, and the like by means of sound and radio waves. M ore particularly it relates to such a system wherein high quality of radiated energy or power is obtained.

It was well known, previous to this invention that energy or power in the form of sound waves could be translated into the form of radio waves by means of suitable translating` apparatus. Such radio wave energy or power could then be re ranslated into sound energy or power by means of suitable receiving apparatus. The resulting sound waves at the receiving station, however, differed materially from the originating sound waves with the result that the received sound was of inferior tone quality. h'lany efforts nave been made to improve the tone quality or faithfulness of reproduction of such systems.

A. high degree of faithfulness of reproduction is especially desirable where the radiated energy is to be retranslated into sound waves of large volume for reception in a room or auditorium without the use of telephone receivers by the auditors. It has been found that a hign degree of faithfulness of reproduction is necessary in order to sound pleasing to the ordinary person and therefore to be satisfactory when the sound waves of music or speech are reproduced by loud speaking receivers. Therefore, a high degree of faithfulness of reproduction is especially desirable where loud speaking receivers are employed in connection with radio receivers.

An object of this invention is to imprcve the faithfulness of reproduction in a radio broadcasting` system by a novel arrangement of energy or power translating elements. Another object is the provision of a high- 1922. Serial No. 566,981.

quality radio broadcasting system by the combination of circuit elements individuall capable of a variety of uses. Still another object is the provision of a radio broadcasting system, the energy from which can be ret-ranslated into sound waves of high tone quality and large volume and still sound satisfactory.

In one embodiment of the invention a radio transmitter is employed to translate energy of audio-frequency into radio energy modulated in accordance with audio-frequency energy. The radio transmitter is designed to receive ancho-frequency energy of substantially the value. of that supplied by an ordinary telephone transmitter. In order to obtain a high degree of faithfulness of. reproduction in the radiated power a high quality source of audio-frequency power is combined with the radio transmitter. This source consists of a high quality transmitter of low power output combined with a distortionless amplifier for increasing this power to substantially the value of that furnished by an ordinary telephone transmitter. An air-damped, stretched-diaphragm, push-pull transmitter is used to translate the power or energy, as it is often called, of sound waves into audio-frequency electrical power of a high degree of faithfulness of reproduction but of low power for use with the distortionless amplifier.

The novel features which are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawings in which Figs. 1 and 2, taken together show diagrammatically the layout and circuit arrangement of one embodiment ofthe invention and Fig. 3 shows the mechanical construction of a suitable high quality transmitter.

Referring t0 Fig. 1 the three blocks A, B, and C enclosed by dot and dash lines, represent respectively a studio where the high quality transmitter is located, a distortionless amplifier panel for amplifying the low power of the transmitter to a value substantially equal to that of an ordinary telephone transmitter and the power room where the necessary batteries and motorgenerator set are located.

Referring to Fig. 2, the blocks D, E, F and G enclosed by dot and dash lines represent respectively a radio transmitter, an antenna relay for switching the antenna from the receive tothe transmit condition and vice versa, a power panel for controlling the motor-generator set, and the radio operators desk.

The transmitter shown within the block A is located in the studio. The apparatus shown within blocks B, D, E, F and G are ordinarily located in the same room which may be designated as the radio room. The batteries and 1notor-generator within the block Care preferably located in a separate room which will be designated as the power room.

As indicated, the high quality transmitter located in the studio at A comprises two carbon buttons 5 and 6, one on either side of a vibrating diaphragm 7. 'lhis type of transmitter is known as a push-pull transmitter. The mechanical design of this transmitter, which produces low power audio-frequency electrical power due to the vibration of its diaphragm in response to sound waves, is shown in Fig. 3.

Referring to Fig. 3 the metallic diaphragm 7 is clamped between two annular rings 151 and 152 which are held together by screws 153 which are arranged at intervals around its circumference. At the point where the rings 151 and 152 contact with the diaphragm 7 they are provided with spiral grooves so arranged as to hold the diaphragm securely, when the rings are clamped together.

In order to stretch the diaphragm a ring 154, which fits into a groove in the ring 151, is provided with a rounded surface where it comes in contact with the diaphragm 7. An annular curved recess is provided in ring` 152 into which the diaphragm 7 is forced by ring 154 under pres sure of a plurality of screws 155 arranged at intervals around the ring 151. The diaphragm is forced into the annular recess of ring 152 by the pressure of ring 154. Another ring 156 having a curved surface, as shown. in contact with the diaphragm 7, is held in place and forced against the diaphragm 7 by ring 157 having threaded engagement on its exterior circumference with ring 152. The pressure of ring 157 against ring 156 further stretches the diaphragm 7 until it has a natural period of approximately 6500 periods per second or higher.

On the opposite side of the diaphragm 7 a plate 158 is provided having a recessed surface 159 slightly separated from the diaphragm 7. A 'series of openings 160 are arranged around the outer circumference of this recessed surface in order to allow the entrance and escape of air when the diaphragm 7 is vibrated. By this means the diaphragm is air-damped and aids materially in the production of high quality audio frequency currents. On either side of the diaphragm 7 are carbon buttons 5 and 6 carried on suitable bridges 162 and 163. Each carbon button is provided with a felt washer 164: which serves to prevent the carbon granules 165 from escaping between the metal portion of the buttons 5 and 6 and the diaphragm 7. The ring 158 is held in place by the clamping ring 166 which in turn is forced toward the ring 151 by means of a plurality of screws 167 arranged at intervals around the plate 166.

The ordinary telephone transmitter, with an average voice talking close, has a power output of about 6 milliwatts; while` the push-pull high quality transmitter, just described, under similar conditions, has a power output of about 6 microwatts or about one onethousandth as large. Under ordinary conditions the speaker', singer orother source of sound waves is located at a considerably greater distance from the transmitter, sometimes as much as 10 or 15 feet, and the power output therefore is ordinarily considerably less than 6 microwatts.

In order to provide for a high degree of faithfulness of reproduction with loud speaking receivers, the natural period of vibration of the transmitter diaphragn'i should not be substantially less than 4000 periods per second and preferably it should be higher than 5000 periods per second.

The distortionless amplifier located on the amplifier panel B comprises three electron discharge devices 8, 9, and 10 connected in tandem. These devices are of the threeelectrode type each comprising a cathode or filament, an anode or plate and an impedance control element or grid. The filaments 11 and 12 of devices 8 and 9, respectively, are connected in series with each other and heated by current from battery 13. In series with the filaments 11 and 12 are fixed resistances 14 and 15, choke coil 16 and a current regulating rheostat 17.

The filament 18 of device 10 is also heatedy from battery 13. In series with the filament 18 are the choke coil 19 and a current regulating rheostat 20. The rheostat-s 17 and 20 are also adapted to open their respective circuits. Y

Space eurents for devices 8, 9 and 10 are supplied from battery 21. In series, respectively, with the anodes 22, 23 and 2l of devices 8. 9 and 10 are choke coils 26 and 27, Iliese choke coils are designed to act as high impe-dances to audio-frequency currents but to be of low impedance to current from the battery 21. Bridged across the battery 2l is a large condenser 28 which in conjunction with the choke coils 25, 26 and 27 prevent interference currents from being impressed on audio-frequency circuits of the amplifier. The choke coils 25, 26 and 27 are of high impedance to alternating current of frequency as low as 60 cycles per second and aid materially in providing distortionless amplification of the audiofrequency currents.

Audio-frequency power is impressed upon the input circuit of device 8 by means of input transformer 29, the secondary winding of which is connected to the terminals of resistance 30. The grid 31 is connected through the two-point switch 32 tol one terminal of this resistance with the switch in the position shown or to an intermediate point in the resistance in the other position. The position of the switch controls the power amplification of the amplifier. The other terminal of the secondary of transformer 29 is connected to the filament heating circuit of device 8 at a point intermediate the resistance 14 and choke coil 16. The input circuit of device 9 including the grid 33 is connected to the output circuit of device 8 by means of a condenser 34 and a potentiometer 35. The input circuit of device 10, including the grid 36, is connected to the output circuit of device 9 by means of condenser 37 and resistance 38. Battery 39 is also included in the input circuit of device 10 to impress a negative potential upon the grid 36 with respect to the filament 18. The output circuit of device 1() comprises condenser 40 and output transformer 4l. The secondary winding of which is connected to conductors 43 and 44.

The plate current for each of the devices 8, 9 and 10 as shown is obtained from the battery 21. Terminals I'I, I, J and II are provided in case it is desired to use a higher plate potential for device 10 than for devices 8 and 9. Instead of connecting the terminals I and J together as shown the terminals I-I and I may be connected together and likewise J and K, in which case resistance 45 acts as a potentiometer connected across the terminals of battery 21, from an intermediate point of which potential for devices 8 and 9 is obtained by way of terminals II and I.

Current for the high quality transmitter at A is obtained from battery 13 by means of resistance 46 which, in conjunction with resistance 47 and choke coil 48, acts as a potentiometer in the common lead from the diaphragm 7 to the mid-point of the primary winding of transformer 29. A switch 49 is arranged to interrupt the Qurrent in the transmitter Qrcuti f y The radio transmitting apparatus comprises an oscillator C, a variable impedance device M, and the thermionic amplifier L. The oscillator O includes two electron discharge devices 50 connected in parallel and cach having the usual cathode, anode and impedance control element or grid. The alternating current output circuit of the devices 50 may be traced from their anodes or plates by way of choke coils 51, 51, condenser 52 and primary winding in parallel, and condenser 54, to the cathodes or filaments. Primary winding 53 is indue-tively coupled to secondary winding 55 included serially in the circuit of conductor 56 which extends to the antenna Primary winding a8, also incl udcd scri ally in the antenna circuit, is inductively coupled to secondary winding 59 and serves to feed. back oscillations from the antenna circuit to the alternating current input circuit of devices 50 which includes winding 59 and capacity element 60 in series. An inductometer winding 61 enables variation of the tuning of thc antenna 57 which serves as the frequency determining circuit for the oscillator' O.

The variable condenser 52 serves to so change the constants of the interior circuit as to prevent oscillations therein. The interior circuit comprises winding 59, condenser 60. condenser 54, elements 52 and 53 in parallel and the inherent capacity between the anodes and control elements. Condenser 52 therefore serves to prevent the absorption of power by interior oscillations. It further serves to improve the power factor of the plate current, in a measure compensating for the leakage reactance of the winding 53.

Modulation is accomplished by means of the constant current system of R. A. I-Ieising fully disclosed at page 360, Proceedings of A. I. E. E.. volume 38, No. 3.

The variable impedance device M comprises two electron discharge devices G2 connected in parallel.

A generator GB having a separately eX- cited field winding FB supplies space current to devices 50 and 62 in parallelover a path including constant current choke 63, .inductance 64, right hand blade of switch 66, and automatically operati-ng circuit breaker 65 to the positive terniinal of generator GB. From the negative terminal of this generator, the space current path may be traced by way of the left-hand blade of switch 66, resistance 67, and resistance 68 to the cathodes of devices 50 and G2. Connected in shunt across the space current circuit are capacity elements 69` which in conjunction with series inductance 64. serve to prevent commutator ripples and other lucv` tuations in the voltage of source GB from affecting the space current and. alSQ Serve to prevent high frequency oscillations from being impressed upon the generator GB through the space current circuit.

In series with the anodes of devices 62 are the high frequency choke coils 87, 87. The choke coils 51, 51 and S7, 87 prevent parastic oscillations in the devices with which they are associated of frequency above the generated radio frequency as described in U. S. Patent 1,437,021, November i8, 1922. to J. C. Schelleng.

Filament heating current is supplied by a generator GA having a shunt field winding FA. In series with the shunt field winding` F A is a variable resistance 70. The separately excited field winding FB of generator GB is connected across the output terminals of generator GA in series with a variable resistance 71 and is supplied with exciting current therefrom. A switch 72 is interposed in the filament heating circuit to enable this circuit to be opened when desired.

The input circuit of thermionic amplifier L is coupled by a transformer 7 3 to the audio frequency conductors 43 and 44. The secondary winding of the transformer 7 3 is shunted by a resistance 74 to improve the impedance characteristic of the transformer 7 3 in well known manner. Amplifier L is supplied with cathode heating current from source GA and is supplied with space current by source GB over a path identical with that for devices and 62 except that instead of passing through the constant current choke 63, the space current for the amplifier traverses a resistance 7 5 to reduce the effective voltage impressed across the electrodes of the amplifier. Resistance 75 is shunted by a capacity element 76 to reduce the impedance of the output circuit of the amplifier for the amplified audio 'frequency currents.

A transformer 77 couples the output circuit of amplifier L with the input circuit of variable impedance device M. A resistance 7S of small magnitude with respect to the input impedance of devices 62 is included in shunt to the secondary winding of transformer 77 to stabilize the impedance connected to this winding.

A radi@ frequency choke 7 9, which freely passes low frequency signal eurent, serves to prevent radio frequency oscillations from being impressed upon and dissipated in the circuits of the variable impedance device M and the space current generator GB. rlhe cathode of each electron discharge device is equipped with an individual regulating resistance 80 for varying the heating current.

The input cir-'suit of amplifier L may be traced from the impedance control element of the amplifier through the secondary winding of transformer 7 3 to the lower terminal of resistance 67 and thence through this rcsistance and resistance 68 to the cathode. Since space current passes through resistances 67 and 68 there will be a potential drop between the cathode and the lower terminal of resistance 67 and the control element or grid of the amplifier L will accordingly be maintained at a potential negative with respect to that of its cathode.

The input circuit of discharge devices 62 is also connected to the lower terminal of resistance 67 and their impedance control elements are accordingly maintained at a potential also negative with respect to their cathodes.

The grid leak path of the oscillator includes a resistance element 81 connected to the upper terminal of resistance 67 and the control elements or grids of the oscillator O are maintained at a less negative potential determined by the potential drop across resistance 68 in consequence of the space current flowing therethrough.

The grids of the amplifier L, variable im pedance device M and oscillator O are accordingly each maintained at such negative potentials with respect to their individual cathodes as to reduce their respective space currents to extremely small values. Oscillations are not produced under these conditions.

A time-limit relay 82 is connected across the terminals of generator GA. This relay when energized closes its normally open contacts S3 to short circuit resistance 68 providing the antenna switch is in the transmit position and the normally open contacts 84 are closed as will be described hereinafter.

As soon as resistance 68 is short circuited the grids of the various discharge devices become less negative and permit more space current to flow, thus rendering each of the devices operative and causing radio frequency oscillations to be produced in the antenna. This action is further increased by the reduction of the external resistance in the space current ycircuit when resistance 68 is short circuited, in effect increasing the effective space current voltage.

In order to prevent surges in the space current circuit in consequence of opening or closing of contacts 83, a path including a resistance 85 and a capacity element 86 in series therewith is shunted across the terminals of resistance 68.

Ammeters 200 and 9.01 are conveniently located to measure the space current of the variable impedance device M and oscillator O respectively. rlhe grid current of the oscillator O and the antenna current are indicated by ammeters 203 and 202 respectively. A voltmeter 204 located on the power panel having its terminals connected directly across the filament heating circuit of the. radio transmitter at D indicates the actual potential drop across the filament.

The antenna 57 is normally connected to a radio receiver, indicated by block 87 and telephone receivers S8 located on the operators desk G, through the upper closed contacts 89 and 90 of the antenna switch at E.

In order to connect the antenna 57 to the radio transmitting apparatus by way of lead 56, the switch 91 is closed. The closure of switch 91 causes current to flow through relay 92 from battery 13 to close the normally open front contacts of relay 92. The closure of the front contact of relay 92 causes current from the low voltage generator GA to flow through the windings 93 of the antenna relay in parallel. The energization of windings 93 of the .antenna relay opens the upper cont-acts 89 and 90 and closes the lower contacts 94 and 95. At the same time, normally open contacts 84 are closed in order to complete the circut for short cir'cuiting resistance 68 upon the closure of contacts 83 of time limit relay 82 for the purpose hereinbefore described. A resistance 9G is connected in series with the parallel connection of windings 93 and, when the short circut thereabout is removed by the opening of contact 97 the holding` current through windings 93 is reduced.

In order that the radio operator may monitor the audio-frequency currents which are impressed upon the input circuit of the radio transmitting apparatus through trans* former 73, a loud speaking` receiver and a volume indfcator are bridged across the input yconductors 43 and 44.

The volume indicatorl may be any suitable instrument for gauging the power output of a circuit carrying audio-frequency currents. The preferred form is a voltage indicating device and consists of an electron discharge device, like device 8, the filament heating and space currents being obtained from the same batteries as those for that device. In series with the filament is an inductance element. identical with inductance element 16, and a filamentrheostat, like rheostat 17 having a range of 0-10 ohms. In place of the resistance 14 is a vari able resistance having a range of 0 to 4 ohms. Input potential is impressed upon the device by means of an input transformer having variable twin ratio. The fixed winding of the transformer is connected across the conductors 43 and 44. In the lspace current circuit is an inductance element, having an inductance value of 250 henrys connected in series with a milliammeter and a 20 ohm resistance in parallel. This inductance element damps the milliammeter. A 2 microfarad ycondenser is direct-ly connected between the plate and filament. The negative potential cn the grid is so adjusted that a current of approximately 0.1A to 0.2 milliampere flows in the plate circuit when no alternating current potential is impressed upon the input circuit. With this adjustment the device acts as a rectifier, of .audio-frequency waves, impressed upon it and the scale deflection is an indication of the power input.

The preferred adjustment is such that the power from an ordinary telephone transmitter will cause about a three quarters scale deflection when the input transformer has a one to one turns ratio. The .same deflection may be maintained for different values of input power by changing the turns ratio and a change in the deflection with the same turns ratio indicates a` change in the power input.

Both the low voltage generator GA and the high Voltage generator GB are located on the same shaft and driven by motor M0. This motor is shunt wound having an armature 100 and a field 101. Power for driving the motor is obtained from suitable power leads which are connected to the power panel F at terminals 102 and 103.

In the arrangement shown, power is obtained from a direct current source and the circu`t to the motor M0 and motor starter is closed through switch 104. The motor M0 is started by means of an automatic starter located on the power panel IF which in turn is controlled by push buttons 13B-1 and PIB-2 located respectively on the power panel F and the operators desk G.

The push buttons PB-l and 13B-2 are connected in parallel and the description of use of one will serve for both. The closure of the upper contact of push button 13B-1 closes a circuit for the upper winding of relay 105 from power terminal. 102 upper blade of switch 104, normally closed contact 106 and upper winding of relay 105, conductor 116, and lower blade of switch 104 to power. The energization of relay 105 causes the closure of both the upper and lewer front contacts followed by the opening of contact 106.

Field winding 101 of motor M0 is energized from the power supply over a circuit which may be traced from power terminal 102, upper blade of switch 104. closed lower front Contact and lower holding winding of relay 105, conductor 107. field winding 101, conductor 108, lower blade of switch 104 to terminal 103. At the same time, current is supplied to the armature 100 of motor M0 by a circuit which may be traced from power terminal 102, upper blade of switch 104` closed lower front contact and low-er holding winding of relay 105, conductor 107. armature 100 of motor M0. conductor 109. resistances 110 and 111. lower blade of switch 104 to power terminal 103.

A circuit is also closed for relay 112 across the terminals of the armature 100 of motor M0 which may be traced from the positive terminal of armature 100, conductor 109,

normally closed lower back contact of relay 112, winding of relay 112, upper closed front contact of relay 105, lower front contact of relay 105, lower holding winding of relay 105, conductor 107, tothe negative terminal of armature Relay 112 is not energized until tte speed of the motor has increased to such a point that the potential drop across its armature is suflicient to operate the relay. 'Vhen this occurs the resistance 110 is short circuited through the front contact of relay 112 and the motor attains a still higher speed due to the removal of resistance 110 from the armature circuit. A short circuit around the winding of relay 113 is removed by the opening ofthe lower back contact of relay 112. The windings of relays 112 and 113 are now connected in series with each other across the terminals of armature 100. The potential drop across the armature 100 is sutlicient to hold theY armature of relay 112 in actuated position but is insuiicient to actuate relay 113 until the motor has attained a still higher speed. lVhen this point has been reached relay 113 is energized and the resistance 111 in the motor armature circuit is short circuited at the upper front Contact of relay 113. The opening of the lower back contact removes a short circuit about resistance 114 which is then connected in series with windings of relays 112 and 113 across the armature 100 of motor M0. The resistance 114 reduces the holding current of relays 112 and 113 when the motor has attained full speed.

After relay has been energized the armatures are held in actuated position partly by the motor current through the lower winding and partly through the energization of the upper winding over a circuit extending from power terminal 102, upper blade of switch 104. lower front contact and lower winding of relay 105, resistance 115, upper winding of relay 105, conductor 116` lower blade of switch 104 to power terminal 103.

In order to shut down the motor the lower contact of push button PB-l or PB-2 is closed, short circuiting the upper winding of relay 105 which is deenergized thereby. The motor circuit is thereby opened at the lower front contact of relay 105 and the energization circuit for relays 11.2 and 113, at the upper front contact of relay 105. Contact 106 of relay 105 is closed and the circuits are again ready for starting the motor, upon closure of the upper contact of push button PB-l or F13-2.

From the foregoing description it is seen that the transmitter which serves as a source of low power audio-frequency waves is located in the studio or transmitting room A. The distortionless amplifier which in creases the power of these audio-frequency waves to substantially that furnished by an ordinary microphone is conveniently located on the amplifier panel B. The radio transmitter which is adapted to receive audio-frequency power of value substantially equal to that furnished by an ordinary telephone transmitter for translation into power of radio frequency is located on a radio transmitter panel D, which is connect-ed to the antenna through an antenna relay at E. The power for both the amplifier at B, radio transmitter at D and the antenna relay at E is obtained from batteries and a motor generator set located in the power room C This energy is centrollcd from a power panel F The motor generator may also be started from the opcrators desk G from which position the antenna relay is also controlled.

The method of operation is as follows: The radio operator closes power switch 104, radio transmitter filament heating circuit switch 72, radio transmitter space current switch 66 audio-frequency transmitter switch 49, filament heating circuits of the amplifier B at variable resistances 17 and 20 which for maximum value of resistance may be open circuited, and the spaced current circuits of amplifier B at terminals H, I, J and K. For reception the radio receiver will be used in well known manner. The switch 91 is closed operating relay 92 and in turn antenna relay E to connect the antenna 57 to the radio transmitting apparatus through contacts 94 and 95 of the antenna relay and conductor 5G. The motor generator set in the power room C is started by closing the upper contacts of either push button PB-l or PIB-2.

Generator GA will thereupon supply heating current to the cathodes of each of the electric discharge devices, energizing cur rent to the time-limit relay 82, and field current to generator GB. Generator GB thereupon builds up its voltage and supplies space current through resistances 67 and 68 to each of the electric discharge devices. After sufficient time has elapsed to permit the cathodes to become fully heated, time-limit relay 82 operates to close contacts 83, short circuiting resistance 68. This decreases the normal negative potential on the grids or control elements of the amplifier L and discharge devices 50 and 62 and permits full magnitude space current to flow through each of these devices. The oscillator thereupon produces radio frequency oscillations in the antenna 57.

Sound waves originating in the studio A are impressed upon the high quality airdamped transmitter located therein and the audio-frequency power of small value delivered by the transmitter is impressed upon the distortionless amplifier B through the input transformer 29. This power is amplified by means of the electron discharge devices 8, 9 and 10 until its value is substantially equal to that furnished by an ordinary telephone transmitter. This amplied power is then impressed upon the input transformer 73 of the radio transmitter at D by means of conductors 43 and 44.

In the preferred embodiment of this invention the high quality stretched diaphragm transmitter has a natural period greater than 6500 periods per second and a power output of about 6 microwatts. The current flowing through each carbon button is approximately milliamperes or less. The input transformer 29 of the distortionless audio-frequency amplifier on panel B has an impedance ratio of 200 to 100,000 ohms, the high impedance winding being connected to the input circuit of device 8. The lower part of resistance 30 has a value of 4,000 ohms while the upper part a value of 100,000 ohms. 'The resistances 14 and 15 in the filament circuit of devices 8 and 9 each have a value of 1.2 ohms. The condensers 34, 37 and 40 in the output circuits, respectively, of devices 8, 9 and 10 each have a value of 1 microfarad. The potentiometer in the input circuit of device 9 has a value of 500,000 ohms while the resistance 38 connected across the input circuit of device 10 also has a value of 500,000 ohms. The battery 39 has a value of 9 volts. An 18 volt battery 13 is used to furnish iilament current for each of the devices 8, 9 and 10 while space current is furnished by a 130 volt battery 21, composed of 6 dry cells connected in series.

In case it is desired to use a higher space voltage for device 10 than for devices 8 and 9, a 350 volt battery composed of 6 dry cells or a suitable motor generator may b e used in place of the 130 volt battery previously mentioned. When the higher voltage source is employed, the battery 39 in the 1nput circuit of device 10 has a value of 22.5 volts. The lower part of resistance has a value of 17 ,000 ohms while the upper part has a value of 24,000 ohms. The choke coils 25 and 26 each have a resistance value of 15,200 ohms and an inductance value of 700 henrys. Choke coil 27 in the space current circuit of the device 10 has a resistance value of 560 ohms and an inductance of 100 henrys.

In series with the iilament heating circuit of devices 8 and 9, retardation coil 16 has a value of 1.66 ohms and-an inductance of 0.1 henry. The rheostat 17 has a value of 6.5 ohms. The retardation coil 19 in the filament heating circuit of device 10 has a value of 1.66 ohms and an inductance of 0.1 henry. The rheostat 20 has a value of 8 ohms. The condenser bridged across battery 21 has a value of 6 microfarads.

The output transformer of device 10 has an impedance ratio of 6000 to 500 ohms, the

high impedance winding being connected to device 10.

Referring now to the radio transmitter shown at D of Fig. 2, for a transmitter having a capacity of 500 watts radiated power at the antenna, the followingl values are suitable.

The input transformer 73 has an impedance ratio of 350 to 50,000 ohms. Connected across the high impedance winding of input transformer 73 is a resistance 74 having a value of 48,000 ohms. The input transformer 77 has an impedance ratio of 6,00() to 6,000 ohms. Connected across the seeondary of input transformer 77 is a resistance 7 8 having a value of 6,000 ohms. The choke coils 51 and 87 consist of 10 turns of No. 22 B and S copper wire wound as a solenoid 1 in diameter. The high frequency choke coil 79 is of negligible resistance and has an inductance of 0.4 millihenrys. The variable condenser 52 has a capacity range of 0 to 1500 microfarads. The fixed condensers 54 and 60 have values, respectively, of .005 and .01 microfarads. The coils 53, 55, 58, 59 and 61 constitute a unitary structure as described in application of J. C. Gargan, Serial No. 524,370, led December 23, 1921. The space current choke coil 63 has an inductance value of 0.75 henrys. Of

the resistances for controlling the grid potentials of the several devices, the resistance 68 has a value of 2,000 ohms, the resistance 85, 200 ohms and the condenser 86, .5 microfarads, while the resistance 67 has a value of about ohms, depending upon the amount of modulator grid potential required.

The potential supplied by low voltage generator GA is from 10 to 14 volts, while the potential supplied by the high voltage generator GB is 1600 volts. The resistance 75 in the plate current circuit of amplifier L has a value of 10,000 ohms and is shunted by condenser 76 of one microfarad. The induetance coil 64 in the plate current supply circuit has an inductance value of 0.75 henrys while the condensers 69 each have 'alue of 1 mierofarad.

For the electron devices of the preferred embodiment of the invention Western Electric Company vacuum tubes are employed, having the following designations:

No. 209-11 for devices 8 and 9 and the volume indicator,

No. 205-13 for device 10,

No. 211%A for amplifier L, and

No. 212-A for devices 50 and 62.

In order to clearly disclose an operative embodiment of the invention, certain design values and combinations of apparatus have been given hereinbefore. It will be obvious to those skilled in the art that other values and combinations may be used to obtain different amounts of output power. The equipment described, however, provides high 'Vf' l. A radio broadcasting system comprising a radio transmitter producing radio power modulated in accordance with audiofrequency power of value substantially equal to that furnished by an ordinary telephone transmitter, an audio-frequency input circuit for said radio transmitter, a push pull air-damped transmitter to translate sound waves into audio-frequency electrical power of relatively small value compared with that furnished by an ordinary telephone transmitter with consequently improved quality, a distortionless audio-frequency amplifier to increase the power urnished by said push pull transmitter to a value substantially equal to that furnished by an ordinary telephone transmitter, and means to impress said amplified audio-frequency power upon said input circuit of said radio transmitter.

2. In combination a constant current radio transmitting apparatus designed to receive audio-frequency power from an ordinary telephone transmitter, an air-damped distortionless transmitter for translating sound waves into audio-frequency electric waves, and a distortionless amplifier for amplifying the low output power of said airdamped transmitter to a value substantially equal to thatof an ordinary telephone transmitter to be impressed upon said radio transmitter.

3. A radio broadcasting equipment comprising in combination a distortionless, airdaniped transmitter, a multi-stage audiofrequency amplifier, and a radio transmitter.

4. A radio broadcasting; equipment, comprising in combination a distortionless airdamped transmitter, a three-stage audio- 'frequency amplifier, and a radio transmitter A radio broadcasting equipment comprising in combination a distortionless airdamped, push-pull, carbon-button transmitter, having a diaphragm whose natural period is higher than 5000 periods per second, a multi-stage audio-frequency amplifier, and a radio transmitter.

G. A radio broadcasting equipment, comprising in combination an air-dannped, double-button, carboli-transmitter having a. diaphragm whose natural period is higher than 5000 periods per second, a muti-stage audio-frequency amplifier having inductan ce coiqpling between stages and a radio transmitter coinprising a constant current modulating apparatus, and one stage of audiofrequency amplification inductively coupled to said multi-stage audio-frequency ampli- 7. In combination with a radio transmitter for generating a radio frequency wave of at least 100 watts and modulating said wave with power of the order of 6 milliwatts, a high quality speech frequency equipment comprising an air-damped transmitter, having a diaphragm whose natural period is higher than 5000 periods per second and having a power output of substantially 6 microwatts or less. and a multi-stage electron discharge amplifier for increasing this power to substantially 6 milliwatts and supplying said amplified energy to said radio transmitter.

8. A radio broadcasting equipment, comprising in combination, a radio telephone tansmitter suitable for operation in conjunction with ordinary telephone lines, and a speech input equipment comprising a transmitter provided with a diaphragm having a natural period of the order of 5,000 periods per second, and a multi-stage distortionless audio-frequency amplifier for supplying power to said radio transmitter.

S). A radio broadcasting equipment comprising a unitary transmitter having a diaphragm whose natural period of vibration is not substantially less than 4000 periods per second, a unitary distortionless amplifier receiving power from said transmitter, and a unitary radio transmitter receiving audiofrequency input power from said distortionless amplifier.

10. A radio broadcasting equipment comprising a unitary transmitter having a diaphragm whose natural period of vibration is higher than 5000 periods per second, a unitary distortionless amplifier receiving power from said transmitter, a unitary radio transmitter receiving audio-frequency input power from said distortionless amplifier, an antenna from which the radio power is transmitted and control circ-nits for said transmitter, amplifier and radio transmitter.

ll. A radio broadcasting equipment, comprising in combination, a distortionless airdamped transmitter, a multi-stage audiofrequency amplifier, a radio transmitter and a loud speaking receiver connected t0 the output of the said amplifier for monitoring the audio-frequency input to the radio traiismitter.

'12. A radio broadcasting equipment, comprising in combination, a radio telephone transmitter suitable for operation in conjunction with ordinary telephone lines, a speech input equipment comprising a high quality transmitter and a multi-stage audiofrequency amplifier supplying power to said radio transmitter, and a loud speaking receiver to which said amplifier also supplies power.

18. A radio broadcasting equipment, comprising in combination, an air-damped, double-button carbon transmitter having a diaphragm whose natural period is higher than 5000 periods per second, a multi-stage audiofrequency amplifier having inductance coupling between stages, a loud speaking recever connected to the output of said amplifier for monitoring purposes and a radio transmitter comprising a constant current modulating apparatus.

14. A radio broadcasting equipment, comprising in combination, a distortionless airdamped transmitter having a diaphragm whose natural period of vibration is not substantially less than 4000 periods per second, a multi-stage audio-frequency ampliier, and a radio transmitter.

15. A radio broadcasting equipment, oomprising in combination, a distortionless airdamped push-pull carbon button transmitter having a diaphragm whose natural period is not substantially less than 4000 periods per second, a multi-stage audio frequency ampliiier and a radio transmitter.

16. A radio broadcasting equipment, comprising in combination a distortionlessairdamped transmitter, a multi-stage audiofrequency amplifier, a radio transmitter and EL @www connected to the output of said amplifier for indicating the audio-frequency power input to the radio transmitter.

17. A radio broadcasting equipment, comprising in combination, a distortionless airdamped transmitter, a multi-stage audiofrequency amplifier, a radio transmitter and a loud speaking receiver and volume indicator connected in parallel tothe Output of said amplifier for monitoring the audiofrequency input power to the radio transmitter.

In witness whereof, I hereunto subscribe my name this sixth day of June A. D., 1922.

JOSEPH P. MAXFIELD. 

